The objective of the proposed research is to determine effects of autonomic mediators on cellular electrophysiologic characteristics of perinatal hearts. Standard microelectrode techniques will be used to first study the ontogeny of cellular electrophysiologic properties of the ventricular specialized conducting system, sinus node preparations, and working myocardium. The effects of cholinergic and adrenergic agonists and antagonists on cardiac electrophysiologic properties will be studied using isolated, superfused preparations obtained from canine fetal hearts during the first, second and third trimesters. In addition, chemical sympathectomy of pregnant animals and of neonatal ones will be performed using 6-hydroxydopamine. In this manner, I hope to determine effects of partial or complete inhibition of sympathetic nervous system development on cardiac cellular electrophysiologic characteristics. The information obtained from the proposed research should be of value not only to our understanding of normal development of cellular electrophysiologic properties but also to our understanding of disturbances in cardiac rate and rhythm which arise as a result of maternal administration of pharmacological agents which affect heart rate and rhythm in the fetus.